Environmental Engineering Reference
In-Depth Information
PURPOSE
FUNCTIONS
Emergency reactor protection system (other
names- reactor trip system, scram system) should
provide safety in the cases, when the systems of
normal operation fail to keep power unit technical
parameters within the specified design limits. The
reasons of this can be a failure of one of the control
system of normal operation main functions or a
specific event, which could lead to technological
process parameters variation, being too fast for
adequate response of systems of normal opera-
tion. In such cases, in order to prevent emergency
and/or accident, an immediate reactor shutdown
may be required (chain reaction termination and
maintenance of the reactor in a safe state), which
is performed by the relevant safety systems per-
forming emergency protection function. Reactor
shutdown may be required also for the mitigation
of the accidents.
The failure of emergency protection function
can lead to nuclear accident with radioactive
release beyond the project specified limits in the
amount exceeding the determined norms of radia-
tion exposure on the personnel, population and
environment. At the same time, false actuation of
emergency protection in the absence of hazardous
conditions leads to substantial economic loses due
to the unit shutdown, which can resume operation
only after the analysis of the actuation reason and
after obtaining permission to start in accordance
with the determined procedures.
In case of some design limits violations or
conditions of normal operation, emergency can be
prevented without the reactor shutdown by reduc-
ing its power (sometimes it is enough to forbid
power increase). These functions are performed
by preventive protection.
Emergency reactor protection
(ERP) provides
fast termination of chain reaction required in
the most severe initial events. Within the sub-
range controlled during fuel loading, and within
start-up subrange the emergency protection is
performed if the design limit for neutron flux
density is exceeded in each of these subranges.
Within the operational subrange, the reasons for
the emergency protection actuation are relative
level increase of neutron power or decrease of
half-life to a specified (maximum or minimum
accordingly) limit. The emergency protection
is also performed in the cases, when any of the
process parameters (pressure over a core, steam
line pressure, coolant temperature, steam generator
water level, or compensator pressure, etc.) reaches
its limit of the set parameter.
For example, if main circulation pumps (MCP)
are switched off, decrease of coolant flow can lead
to the rise of its temperature, start of vigorous boil-
ing and heat exchange crisis, which, in turn, leads
to fuel elements damage. Decrease of pressure over
core, caused by pressure compensator malfunc-
tioning, can lead to the same result. To accelerate
the response to such violations and to prevent
hazardous consequences emergency protection
actuation is provided in case of safe operational
conditions violations (main circulating pumps
deenergizing, loss of primary coolant circuit,
power outage on power rails, failure of the system
to perform emergency protection function and in
the other cases under the design). For example,
when switching off two of four operating MCPs
(at a power more than 75% of nominal power)
or one of two operating MCPs (at a power more
than 5% of nominal power) the reactor should be
shutdown before the system detect the raise of a
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